This observational study was conducted to design and test a physiologic definition for bronchopulmonary dysplasia at 36 weeks of life. Infants were studied in a supine position with the pulse oximeter in position with good signal prior to collecting baseline data. Feedings and medications were given 30 minutes before the evaluation. Baseline data was collected on infant's current oxygen. Then, the infants were weaned to room air for 30 minutes. If saturations remain ≥90%, the infant was considered to have passed the oxygen reduction challenge (to NOT have BPD). The infant should then be placed back in his/her baseline oxygen. If the infant has saturations <90% for 5 continuous minutes or <80% for 15 seconds, the infant should be immediately placed back in his/her baseline oxygen, and the infant was considered to have NOT passed the challenge (to have BPD).